Microparticles labeled with fluorescent dyes have found use in a wide variety of applications. Fluorescent microparticles are most commonly used in applications that can benefit from use of monodisperse, chemically inert particles that emit detectable fluorescence and that can bind to a particular substance in the environment. The high surface area of microparticles provides an excellent matrix for attaching molecules that selectively bind to targets, while the fluorescent properties of these particles enable them to be detected with high sensitivity. They can be quantitated by their fluorescence either in aqueous suspension or when captured on membranes.
Many luminescent compounds are known to be suitable for imparting bright and visually attractive colors to various cast or molded plastics such as polystyrene and polymethyl methacrylate. Uniform fluorescent latex microspheres have been described in patents (U.S. Pat. No. 2,994,697, 1961; U.S. Pat. No. 3,096,333, 1963; Brit. Patent 1,434,743, 1976) and in research literature (Molday, et al., J. CELL BIOL. 64, 75 (1975); Margel, et al., J. CELL SCI. 56, 157 (1982)). A related patent application of one of the inventors (Brinkley, et al., Ser. No. 07/629,466, filed Dec. 18, 1990) describes derivatives of the dipyrrometheneboron difluoride family of compounds (derivatives of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) as useful dyes for preparing fluorescent microparticles. This family of dyes possesses advantageous spectral data and other properties that result in superior fluorescent microparticles.
Although dipyrrometheneboron difluoride labeled materials are highly fluorescent and photochemically stable, a disadvantage of these fluorescent materials is their relatively small Stokes shift (the difference between the peak excitation and peak emission wavelengths) when only one dye is used. Because the optimum wavelength of the exciting light is close to the peak emission light, fluorescent particles with small Stokes shifts require precise excitation and emission filters to eliminate or reduce interference. The customary use of excitation filters blocks part of the excitation and emission light that would otherwise increase the efficiency of the fluorescence and reduces the intensity of the fluorescent signal. Fluorescent materials that incorporate bright fluorescent dyes with increased Stokes shifts permits maximum utilization of the available excitation and emission light, resulting in a greater fluorescent signal (see e.g. FIGS. 2A and 2B).
Another advantage of fluorescent materials with large Stokes shifts is that they can be more easily detected in the presence of other fluorescent materials. Immunoassays are typically carried out in body fluids which contain many endogenous fluorescent molecules, such as bilins, flavins and drugs. Since the vast majority of interfering fluorescent materials have relatively short Stokes shifts, the use of a fluorescent label that emits at a wavelength far greater than its excitation wavelength makes the label easier to distinguish from background fluorescence, since its fluorescent signal is emitted at a wavelength at which most background fluorescence is minimal.
A third advantage of fluorescent materials with large Stokes shift is their usefulness in detecting multiple analytes in a single sample using a single excitation wavelength. Using two or more different fluorescent labels, each of which can be excited at a particular wavelength (e.g. the 488 nm argon laser principal emission), the emission peaks of the different labels are detected at different wavelengths, where each emission spectrum is characteristic of a single analyte. In order to successfully accomplish this, the emission peaks of the fluorescent labels must be well-separated from each other so the correction factors between the various dyes are minimized. High photostability of the label is also beneficial. Fluorescent materials with a large Stokes shift can be used in combination with fluorescent materials with a smaller Stokes shift where both materials excite at the same wavelength, but emit at different wavelengths, giving multiple signals that can be resolved using optical filters or monochromators.
Unfortunately, fluorescent compounds useful as labeling reagents that have Stokes shifts of 50-100 nm as well as high fluorescence efficiency and emission wavelengths of greater than 500 nm required for detectability are relatively rare. (Haugland, Fluorescein Substitutes for Microscopy and Imaging, OPTICAL MICROSCOPY FOR BIOLOGY pp. 143-57 (1990). The magnitude of the Stokes shift in fluorescent dyes has been found to be generally inversely proportional to the high absorbance needed to ensure a strong signal. Fluorescent dyes in use as labeling reagents for biological molecules, such as xanthenes, dipyrrometheneboron difluorides, rhodamines and carbocyanines commonly have Stokes shifts of less than about 30 nm.
The lack of suitable fluorescent dyes with large Stokes shifts has led to the development and use of protein-based fluorophores known as phycobiliproteins as labels (e.g. U.S. Pat. Nos. 4,520, 110 and 4,542,104 both to Stryer, et al. (1985)). Like other fluorophores, they have been covalently attached to beads and macromolecules. See, e.g., Oi, et al., J. CELL BIO. 93, 981 (1982). These large bilin-containing molecules have the disadvantage of poor chemical stability, instability to photobleaching, limited long wavelength emission capability, bulky molecular size (MW&gt;100,000 Daltons) and relatively high cost. Furthermore, only a few proteins of this type are known and one cannot select or appreciably adjust their spectral properties. In an effort to improve the fluorescent emission efficiency of phycobiliproteins without significantly increasing their molecular size, phycobiliproteins have been covalently coupled to the fluorescent dye Azure A (U.S. Pat. No. 4,666,862 to Chan (1987)).
It is known that covalent coupling of a pair of fluorophores results in a fluorescent dye with a larger Stokes shift than either of the individual dyes (e.g. Gorelenko, et al., Photonics of Bichromophores Based on Laser Dyes in Solutions and Polymers, EXPERIMENTELLE TECHNIK DER PHYSIK 37, 343 (1989)). As with the phycobiliproteins, this approach, although reportedly effective in increasing the Stokes shift, requires complex synthetic procedures to chemically couple the two dyes together and are limited by the number and location of available reactive sites. Furthermore, covalently linked molecules typically have sufficient freedom of movement that significant collisional deactivation occurs, leading to loss of energy by vibrational relaxation rather than by fluorescence. There is a need for a way of combining the spectral properties of dyes by methods other than complex covalent coupling to provide useful fluorescent labels with an enhanced effective Stokes shift.
Energy transfer has been demonstrated between dyes that have been coupled to macromolecules to study intramolecular distances and conformation in biomolecules, e.g., Jullien & Garel, BIOCHEM. 22, 3829 (1983); Wooley, et al., BIOPHYS. CHEM. 26, 367 (1987); and in polymer chains and networks, e.g. Ohmine, et al., MACROMOLECULES 10, 862 (1977); Drake, et al., SCIENCE 251, 1574 (1991). Energy transfer with resultant wavelength shifting has also been described for mixtures of dyes in lasing solutions, e.g. Saito, et al., APPL. PHYS. LETT. 56, 811 (1990). Energy transfer has been demonstrated between monomolecular layers of dyes and other organized molecular assemblies, e.g. Kuhn, Production of Simple Organized Systems of Molecules, PURE APPL. CHEM. 11, 345 (1966), abstracted in CHEM. ABSTRACTS 66, 671 (1967); Yamazaki, et al., J. PHYS. CHEM. 94, 516 (1990). Energy transfer between paired donor and acceptor dyes has also been demonstrated in polymer films as a way of studying the energy transfer dynamics, e.g. Mataga, et al., J. PHYS. CHEM. 73, 370 (1969); Bennett, J. CHEM. PHYSICS 41, 3037 (1964). Although the conformity of research results to Forster's theoretical formulation have been widely reported, utilitarian applications of the theory have been limited. The cited references neither anticipate nor suggest fluorescent microparticles incorporating a series of dyes to be used as labeling reagents with an enhanced effective Stokes shift.
The fluorescent microparticles used for the present invention are described generically and specifically in the parent application Ser. No. 07/882,299 (incorporated by reference) published as PCT International Publication No. WO 93/23492 on Nov. 25, 1993. Additional microparticles are described specifically in U.S. Pat. No. 5,433,896 to Kang et al. (1995) titled DIBENZOPYRROMETHENEBORON DIFLUORIDE DYES (incorporated by reference). Specifically in the co-pending application DIBENZOPYROMETHENEBORON DIFLUORIDE DYES (incorporated by reference), filed of even date herewith by inventors Kang and Haugland. These microparticles, when used with a surface material that is selective for target molecules, provide labels having excitation and emission characteristics that are considered to be the most useful for the detection of specified target molecules or combinations of target molecules.